feat(tui): theme switcher with preview

[xHeaven]

Previously, to see a theme in action, you had to do this:

1. /themes
2. Select a new theme
3. Save with Enter
4. Decide whether you like it or not
5. If you like it, profit, if you don't, go back to step 1

After this PR, the new behavior is this:

1. /themes
2. Preview the available themes via the up and down arrow keys
3. Press Enter if you like a theme to save it or Esc to discard the selection and close the modal

[Copilot]

Pull Request Overview

This PR introduces live theme previews in the TUI theme selector and ensures modal close commands are propagated for cleanup or animations.

• Propagate modal.Close() commands in appModel.Update to enable proper modal teardown.
• Implement live theme preview on list navigation and restore the original theme if the dialog is closed without applying.
• Track and restore the original theme state in the theme dialog.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
packages/tui/internal/tui/tui.go	Return modal.Close() command on ESC and CloseModalMsg cases
packages/tui/internal/components/dialog/theme.go	Add originalTheme/themeApplied, preview on navigation, and restore on close

Comments suppressed due to low confidence (2)

packages/tui/internal/components/dialog/theme.go:84

• Live preview and restore behavior aren't currently covered by tests. Add unit tests to verify theme preview on navigation and theme restoration when cancelling.

	_, prevIdx := t.list.GetSelectedItem()

packages/tui/internal/components/dialog/theme.go:90

• [nitpick] Calling theme.SetTheme on every selection change may cause UI flicker or slowdowns. Consider debouncing updates or applying them only after a brief pause in navigation.

	if item, newIdx := t.list.GetSelectedItem(); newIdx >= 0 && newIdx != prevIdx {

[xHeaven]

^ Sorry for the AI brainfart above, I accidentally left it enabled. 👀

[adamdotdevin]

nice! thanks!